Physics · Laws of motion · chapter 4 of 14

Class 11 Physics Chapter 4: Laws of Motion Assertion-reason questions with answers

2 assertion-reason questions on Laws of Motion, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): For the same weight, rolling friction is much smaller than sliding friction. Reason (R): The laws of friction are empirical relations that are only approximately true.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Both are true statements from the chapter. But the smallness of rolling friction comes from the single, momentarily deformed point of contact that has no relative motion, not from the laws being empirical; R does not explain A.

Question 2
Assertion (A): The weight of a book lying on a table and the normal force on it by the table form an action–reaction pair. Reason (R): For a book at rest on a table, the weight and the normal force are equal and opposite.
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Answer: (d) A is false, but R is true.

R is true: the book is at rest, so by the first law the two forces balance. A is false: both forces act on the same body, the book, and an action–reaction pair always acts on two different bodies. The reaction to the weight is the book's pull on the earth.

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